There's a vast stylistic difference between Jay Leno, 62, and Fallon, who will be several months shy of his 40th birthday when he ascends to the "Tonight Show" throne.

NEW YORK — If Jimmy Fallon was feeling stressed about his impending takeover of "The Tonight Show" during a rehearsal last Thursday at 30 Rockefeller Plaza, he certainly wasn't letting it show.

Granted, the head-to-toe chicken costume he was wearing for an "all-clucking" version of the Lumineers' hit "Ho Hey" made it hard to divine his mood.

"Do you mind doing it one more time?" he asked his fellow "Chickeneers" — country singer Blake Shelton, "Parks and Recreation" star Nick Offerman and "Late Night" writer Chris Tartaro — after two run-throughs.

Fallon's diligence paid off: The broadcast version of the song went viral when it hit the Web late last week. It was exactly the kind of musically driven, pop culture-savvy bit that has become Fallon's trademark in his four years at "Late Night" and one that hints at the changes ahead for the aging "Tonight Show" franchise when he officially replaces Jay Leno in 2014.

Although the two hosts have so far maintained a public show of unity and mutual admiration, there's a vast stylistic difference between Leno, 62, and Fallon, who will be several months shy of his 40th birthday when he ascends to the "Tonight Show" throne.

In his two decades-plus at NBC, Leno has consistently led the ratings — mostly, his critics claim, by playing it safe with middle-of-the-road jokes and softball interviews. His best-known recurring segments — like "Jaywalking," in which he quizzes clueless tourists in L.A., and "Headlines," a collection of unintentionally funny newspaper clippings submitted by viewers — are popular without being particularly innovative.

"A problem a lot of 'cooler' comedians have is they play well on the coasts. Leno tours Middle America every weekend. He aims for people who aren't necessarily comedy fans," said Adam Frucci, editor of the comedy website Splitsider.

Fallon is from a younger generation of comedy nerds who tend to eschew Leno's vanilla sensibility, but he's also not as brainy or absurdist as his "Late Night" predecessor Conan O'Brien. And there is something old-fashioned about his skills as a mimic, making him broadly accessible and well-primed to younger viewers who crave mash-ups and memes.

According to Nick Bernstein, a former executive at NBC now overseeing a talk show hosted by comedian Pete Holmes due this fall on TBS, there's an adage that late-night hosts must use everything they've learned when they're on the job.

As a trained musician, skilled mimic and "Saturday Night Live" veteran, Fallon has a wider skill set than Leno, who is purely a stand-up. "One of Jimmy's great strengths is he's a really top-shelf sketch performer," Bernstein said. "It's rare in the history of late night for anyone to show the ability he has to play other characters, to parody celebrities or politicians."

Since he began hosting "Late Night" in 2009, Fallon has injected new life into a stagnant genre. Backed by the Roots, arguably the best (and easily coolest) house band in network TV, Fallon has made music an essential element of the show and its online success. Their version of "Call Me Maybe" played on children's instruments has racked up more than 13 million views on YouTube.

Although NBC has not yet said whether the Roots will be joining Fallon at 11:35, it seems unlikely that he'd want to continue without them. They do far more than provide a well-timed rimshot or two; they are often part of the joke, in bits such as the improvisational "Freestylin' With the Roots."

Fallon also does something that's become increasingly difficult in this tabloid-obsessed era: He gets celebrities to let down their guard with pleasantly wacky parlor games with such self-explanatory names as Egg Russian Roulette or Rock, Paper, Scissors, Pie. They provide easy laughs while also getting stars to go beyond the usual canned publicity spiel.

Traditionally, the move from "Late Night" to an earlier time slot has required hosts to rein in their more eccentric impulses for a broader audience, but that may not be necessary for Fallon. Though younger-skewing than Leno's, his comedy is rarely hard to "get," unlike, say, O'Brien and his "Masturbating Bear."

"He'll have a bigger budget and a bigger time slot," Frucci said. "I'd like to see him take it further rather than scaling it back. He's clearly doing something right."